Passive load pull systems have been widely used to characterize microwave devices. Load pull involves measuring a Device Under Test (DUT) under controlled conditions, including controlled impedances seen by the DUT. The controlled impedances may include the impedance on any port of the DUT, and they may be at the fundamental operating frequency or at a harmonic frequency. A typical load pull measurement would measure the DUT performance at multiple impedances to show the effect of impedance on the DUT performance. Some other conditions that may be controlled and/or varied include frequency, power level, bias values, or temperature.
In this document, impedance, reflection, or reflection coefficient are all used as general terms to describe the RF termination seen at an RF port. They are functions of the signal coming out of an RF port and the signal at the same frequency coming into the port. Reflection coefficient is related to impedance by the expression
  Z  =            (              1        +        Γ            )              (              1        -        Γ            )      where Z is the impedance and r is the reflection coefficient. Both terms contain the same information, so that if one is known, the other is also known. Therefore, in this document they will be used interchangeably. Also, the terms “RF port” and “reference plane” are used interchangeably in the context of impedance control.
Impedance tuners are commonly used in load pull measurement systems working at radio frequency (RF), microwave (MW) frequencies, and millimeter-wave frequencies. In this document, RF will mean the entire spectrum of frequencies, including microwave frequencies, millimeter-wave frequencies, and higher.
An impedance tuner may include a transmission line, such as a slabline, coaxial line, or waveguide line. Placement of capacitive objects such as probes along the transmission line alters the impedance or electronic profile seen by the DUT which is connected or coupled to the tuner transmission line. The object may be moved axially along the transmission line to affect the phase, while movement of the object transverse to the transmission line will alter impedance magnitude or reflection effects. In automated tuners, motors are used to position the capacitive objects along the transmission line and transverse to the transmission line.
A “passive RE component” does not require energy to operate, except for the available AC circuit it is connected to. A “passive RF module” is incapable of power gain and is not a source of energy. A passive impedance tuner is one where the RF portion of the tuner consists entirely of passive components, so that the tuner itself is also a passive component.
The RF portion of a tuner is the portion containing the RF signal paths, in which RF signals may be present. Passive impedance tuners may control impedance by moving passive objects, such as a capacitive probe, in a passive transmission line. They may also control impedance with solid state switches, which are passive components. A “tuner state” is one specific hardware setting of the tuner which affects the RF portion of the tuner.
Active tuning load pull systems have also been used, but not as widely because of the complexity and cost. Active tuning provides some advantages, including capability to present a higher reflection coefficient than is possible with a passive tuning system, even with fixture losses or other circuit losses. The impedance seen by the DUT can be all the way to the edge of the Smith chart, and even outside the Smith chart, if desired.
In this document, a “tuner system” will refer to a RF measurement system which uses some kind of tuner or tuners to control impedance at a reference plane or planes, e.g. an impedance seen by a DUT.
An “automated tuner” may be computer controlled; a “manual tuner” is controlled manually by the user.
A “passive tuner” controls the impedance at a reference plane with a passive reflection. This means that it reflects a portion of a signal coming out of a port back into that port. It controls the magnitude or phase of the reflected signal by changing RF hardware settings. The maximum reflection is limited by the physical hardware and losses between the tuner and the DUT reference plane.
A passive tuner may be automated with electronic control, but the control circuits are not part of the RF signal path(s). Therefore, electronic control circuits, electric motors, electronic interfaces, and any other aspect of the tuner outside of the RF portion of the tuner do not change the fact that the tuner is passive.
An “active tuner” controls an impedance at a reference plane by feeding a signal back to that reference plane with a specific magnitude and phase relative to the signal from that reference plane. In the context of conducting measurements on a DUT, the active tuner controls the impedance seen by the DUT by feeding a signal back to the DUT with a specific magnitude and phase relative to the signal from the DUT. It would normally use a signal that is either generated or amplified external to the DUT. The active tuner is said to be operating, or controlling the impedance, at the frequency of the “active” signal. In principle, the maximum effective reflection can be up to or even greater than unity. In practice, this is limited by the amount of power generated by the measurement system that can be fed back to the DUT to synthesize that impedance.
A “passive load pull system” means a measurement system using passive tuners, with no active tuners.
In load pull measurements, a signal is applied to or coming from the DUT. Sometimes this signal can be a CW signal, which means that it contains only one fundamental frequency. But sometimes, the signal is a modulated signal, which means that it may contain many frequencies, spread out over the modulation bandwidth. Modulation is important, because some form of modulation is required for a signal to contain or transmit any information. Therefore, all normal signals used in radios or wireless applications will be modulated.
In the prior art, passive tuners have been characterized (or calibrated) by measuring the tuner's 2-port s-parameters at a fundamental center frequency. From the 2-port s-parameters, the 1-port reflection coefficient seen by the DUT can be known. In some cases, the tuner calibration may consist of measuring the 1-port reflection coefficient at the fundamental frequency directly. Although one tuner calibration may cover many fundamental frequencies, they are only used one at a time.
If the signal used to test the DUT was a modulated signal, the known reflection coefficient would be correct at the center frequency of the modulation, but not at other frequencies within the modulation bandwidth. Likewise, de-embedding of measured data from an instrument through the tuner would also be accurate at the center frequency of the modulation but not at other frequencies within the modulation bandwidth. In the prior art, this deviation was ignored when using passive tuners, because it was generally a small effect when the modulation bandwidth was small.
In recent years, wideband modulation bandwidths are being used to transmit high amounts of data. This can cause significant accuracy problems with passive load pull measurements, because the wide bandwidth makes the impedance deviation over the bandwidth to be significant.
In the prior art, this problem of measurement error due to impedance deviation over a wide bandwidth has been solved with active tuning, but this requires a much more complex and expensive system than a passive tuner system.
This invention solves this problem of measurement errors due to impedance deviation over a wide modulation bandwidth when using passive tuners.